Next Level IC-735 Series; Part 1: Smoother Than Stock — Japanese Techniques for Reducing Phase Noise in the Icom IC-735
Every time I restore an Icom IC-735, I’m reminded of how good this radio can sound—until I place it side by side with a modern transceiver or a second IC-735 that’s been thoroughly optimized. That’s when the differences become obvious: a subtle hiss behind CW tones, a slightly fuzzy edge to SSB audio, and digital modes that fail to decode cleanly even under strong signals.
These quirks are common enough that many operators dismiss them as “just the way the IC-735 sounds.” But Japanese amateurs, with their usual attention to detail, have long known otherwise. Several of the most effective upgrades target one often-overlooked source: phase noise from the PLL and VCO sections.
This article is the first entry in my Next Level upgrade series. It builds directly on the work of JA3XEF, who carefully traced this phase noise problem to a few simple but critical design weaknesses. His proposed solutions are elegant, non-destructive, and easy to implement. I’ve tested them myself and extended the concept even further.
They work. Here’s how to apply them.
Understanding the Root Cause: Phase Noise from Power Coupling
The IC-735 uses a PLL synthesizer with four voltage-controlled oscillators (VCOs) to generate the frequencies used for mixing and signal generation. These oscillators are stable in concept but quite sensitive to power supply ripple and internal RF coupling—especially in VCO4, which handles the highest HF bands.
JA3XEF identified two key problems:
Ripple and noise on the regulated 9V supply rail entering the PLL and VCO sections
Stray RF coupling between control lines and the tank coil labeled L19 inside VCO4
Together, these issues create low-level phase noise that affects both clarity and stability. The symptoms are subtle, but they’re real:
CW tones have a soft background hiss
Digital modes (especially FT8, PSK, RTTY) show blurred symbol edges or reduced decoding accuracy. Decodes as < … >.
SSB audio can sound slightly smeared or indistinct
On higher bands (10, 12, 15 meters), PLL lock can be delayed or unstable
Even after a full recap, I’ve found that some IC-735s still suffer from this problem—particularly those that were previously operated under high heat or with aging regulators.
The Fix: Power Filtering and Local Shielding
What follows are three carefully chosen upgrades. They’re easy to reverse, involve no trace cutting, and can be performed with standard tools and basic soldering skills.
1. Decouple the PLL Controller ICs
Install 10 nF ceramic capacitors (C0G dielectric) directly across the power pins of IC4 and IC5, the PLL controller chips on the PLL board. You’ll need to diagonally straddle the chips to directly attach the capacitor to Vdd and Vss pins.
Place the capacitors as close to the pins as possible for best effect
This reduces high-frequency noise on the logic supply lines, minimizing jitter during startup
On one rebuild, this simple step made a measurable difference in PLL lock time and improved signal clarity during warm-up—even without touching any other circuit.
2. Replace R108 with a Ferrite Bead and Capacitor
Locate R108, a 100Ω resistor on the PLL board that feeds the regulated 9V supply into the VCO block. It’s located just outside the shielded VCO enclosure. This is your insertion point.
Remove R108 and replace it with an axial ferrite bead, such as Fair-Rite 2773002112
This suppresses high-frequency switching noise from the regulator, preventing it from reaching the VCOs
Use heatshrink for strain relief and to avoid shorting
Do not add R108 in series — the ferrite replaces it entirely for cleaner RF performance
Optional (recommended):
Add a 10 nF ceramic capacitor (C0G dielectric) from the VCO side of the bead to the nearest ground pad. This forms a simple LC low-pass filter that attenuates remaining ripple. In several test cases, this combination produced a measurable drop in phase noise on the 10–20m bands.
What I observed:
Cleaner FT8 and RTTY signals
Quicker and more stable PLL lock behavior, especially from cold starts
Slight but audible reduction in background hiss on CW
3. Shield the L19 Tank Coil Inside VCO4
This is the most significant improvement. JA3XEF’s original recommendation was to use copper foil tape to wrap and ground the L19 coil in VCO4. I expanded on this idea with a more robust solution:
Fabricated a small three-sided copper tent from a thin old business card wrapped in copper foil
Soldered it directly to a nearby ground pad for solid connection
Applied grounded copper tape to the inside of the VCO can to prevent unwanted RF ingress
On one high-band “dead” unit—a radio that consistently failed to lock above 20 meters—this modification completely resolved the issue. Lock was immediate, frequency stability was restored, and receive tone clarity improved noticeably.
JA3XEF summarized it best:
“Unwanted leakage and resonance can affect signal quality. Even minimal countermeasures improve intelligibility.”
What You Can Expect from These Upgrades
These are not dramatic, showy mods. There’s no glowing LED or display hack. But the performance improvements are real—especially when combined with a full recap and alignment.
Here’s what I observed across several units:
CW tones became more stable and hiss-free
Digital modes decoded more reliably, especially under weak-signal conditions
PLL lock behavior was faster and more predictable, even from cold startup
Overall receive audio had more definition, especially on upper HF bands
On one test unit, adding the copper tent around L19—along with the ferrite bead and decoupling caps—restored PLL functionality above 15 meters and eliminated spurious noise on FT8 receive traces.
This video is just over 1:30. These are the real world differences between a restored IC-735 and a stock IC-735. Judge for yourself whether the effort is worth the time and money.
Parts List (All Digikey-Compatible)
Capacitors:
(2x) VJ1812Y103KXXCW1BC — 10 nF, 100 V, radial-lead, C0G ceramic
(1x) VJ0805Y103KXXCW1BC — 10 nF surface-mount ceramic (optional LC filter to ground)
Ferrite Bead:
(1x) 2773002112 — Axial ferrite bead, 130 ohms @ 100 MHz, Fair-Rite
Copper Shielding:
Use 3M 1181 adhesive copper foil tape with conductive adhesive
Alternatively, use a thin sheet of copper foil and soldering tools
Recommended Tools
Fine-tip temperature-controlled soldering iron
Magnifier or head-mounted loupe
Metal shears or scissors (for shaping copper shield)
Heatshrink tubing or silicone sleeves for ferrite
Digital multimeter (to confirm continuity after install)
Why These Fixes Matter
This kind of work is what makes restoring the IC-735 so rewarding. You’re not just replacing old parts—you’re improving the radio’s performance by eliminating weaknesses that the original design couldn’t fully address.
In the 1980s, engineers didn’t have access to modern test equipment or today’s understanding of phase noise and shielding techniques. But now we do. With a few hours of work and some careful attention to detail, you can take a well-built 1980s rig and make it compete—sonically and electrically—with far newer designs.
Coming Up Next
In the next article, we’ll explore Russian AGC modifications that can dramatically improve the IC-735’s ability to handle strong signals, pileups, and dense digital-mode environments. These changes are subtle but effective—and well worth considering if you regularly operate in contests or on crowded bands.
Support This Project
If this guide helped you quiet your IC-735 or brought it back to life, consider supporting the series. I don’t need twenty-five dollars—just one. One dollar helps cover the cost of parts, tools, and time spent testing these mods so I can share them freely with the community.
👉 Buy Me a Coffee (not a subscription, just a one-time thank-you)
Tags: Icom IC-735, Ham Radio Mod, Phase Noise Fix, Vintage Transceivers, PLL Upgrade, VCO Shielding, Ferrite Bead Mod, HF Radio Restoration, Digital Modes, CW Audio Clarity